Using ODBC to Dynamically Query Your Data Stores


This article was contributed by Tom Archer.

There are times when, as a programmer, you might be faced with scenarios where you do not know
the schema of a database until runtime. Examples of this are ad-hoc query and reporting tools.
In both cases, the end user is allowed to build their own SQL from a list of tables. As you may
already know, it is extremely easy to pass ODBC an SQL string, have it executed, and
retrieve the resulting data. But, how can you do this when you don’t know what the resulting data
will look like when you write your application?

Luckily ODBC provides several functions that can be used for this very purpose. After connecting
to the data source, the next steps needed would be the following:

  • 1. Prepare the SQL statement via the SQLPrepare function.
  • 2. Execute the SQL statement with the SQLExecute function.
  • 3. Call SQLNumResultCols to find out how many columns were returned in the result set.
  • 4. For each column, call the SQLDescribeCol function to get the column type.
  • 5. For each column, convert the SQL type returned from SQLDescribeCol to a C type.
  • 6. For each row in the result set, allocate memory for the data (depending on the C type).
  • 7. For each row, call SQLGetData to read the data into the allocated memory for that row/column.

Did I say “luckily”? Actually, I said it with tongue firmly planted in cheek. Therefore, in this article
I submit to you a class (CODBCDynamic) that reduces the 400+ lines of code required to
fully implement the functionality listed above to 2 lines of code! Here are some examples of how to
use the CODBCDynamic class.

Examples of how to use the CODBCDynamic class

While this article also includes a full-fledged test application, it’s always nice to be
able to see what you’re getting before you invest the time in downloading, unzipping and
running someone else’s code. Therefore, here are some code snippets that show how easy the
CODBCDynamic class is to use.

  • Submitting an SQL statement
  • To submit an SQL statement, you simply instantiate a CODBCDynamic object
    (passing a valid DSN) and then call the CODBCDynamic::ExecuteSQL member function
    (passing the SQL string to execute). That’s it!


    // simply specify the ODBC DSN in the c’tor
    // and pass the desired SQL to the ExecuteSQL function…
    CODBCDynamic odbcDynamic(_T(“YourDsn”));
    odbcDynamic.ExecuteSQL(_T(“SELECT * from OrderHeader”));

  • Retrieving data from a result set
  • In the first example above, I showed you how the CODBCDynamic class allows
    you to submit an SQL statement using the ExecuteSQL member function. However, there
    are times, when your application will only have the HSTMT to a result set. For example,
    if you call the ODBC SDK function SQLGetTypeInfo, you will receive a result set
    with the returned data. Using the CODBCDynamic class, you can read the data into its
    member variables with the following two lines of code.


    // call a function that returns an hstmt to a result set (e.g., SQLGetTypeInfo)
    odbcDynamic.FetchData(hstmt);

  • Retrieving all rows and columns of data once ExecuteSQL or FetchData has been called
  • Once either the ExecuteSQL or FetchData member functions
    have been called, the resulting data can be retrieved from the
    CODBCDynamic object in a very generic manner. The CODBCDynamic
    class has a templatized array (m_ODBCRecordArray) that represents each of
    the records that were read. Each entry in the m_ODBCRecordArray is a templatized
    CMapStringToPtr map of columns and their respective values for that record.
    The map is keyed by the column name (retrieved automatically) and the data is in the form of
    a CDBVariantEx object. However, you never have to worry about such
    technicalities. Assuming that you’ve already called ExecuteSQL or
    FetchData
    , here’s an example of how easy it is to iterate through the returned
    records of an SQL statement.


    // instantiate a CODBCDynamic object (specifying the desired DSN)
    CODBCDynamic odbcDynamic(_T(“Forms Express System Database”));
    // execute the desired SQL
    odbcDynamic.ExecuteSQL(_T(“SELECT * from UserMaster”));

    // retrieve the record array
    CODBCRecordArray* pODBCRecordArray = &odbcDynamic.m_ODBCRecordArray;

    CString strInfo;

    // for every returned record…
    for (int iRecord = 0; iRecord < pODBCRecordArray->GetSize(); iRecord++)
    {
    CODBCRecord* pODBCRecord = (*pODBCRecordArray)[iRecord];

    POSITION pos;
    CDBVariantEx* pvarValue;
    CString strColName;

    CString strValue;

    // for every column within the current record
    for (pos = pODBCRecord->GetStartPosition(); pos != NULL;)
    {
    pODBCRecord->GetNextAssoc(pos, strColName, pvarValue);
    pvarValue->GetStringValue(strValue);

    strInfo.Format(_T(“Record: %ld, Column: %s, Value: ‘%s'”), iRecord, strColName, strValue);
    AfxMessageBox(strValue);
    }
    }

  • Retrieving specific columns once ExecuteSQL or FetchData has been called
  • As mentioned above, once the ExecuteSQL or FetchData function has returned, each
    returned record is stored in an array and each record is a basically a map of
    column names to CDBVariant values. Therefore, as easy as it is to iterate through
    all the returned the data, you can just as easily request specific columns by
    name. Here’s an example of how you would do that.


    // instantiate a CODBCDynamic object (specifying the desired DSN)
    CODBCDynamic odbcDynamic(_T(“Forms Express System Database”));
    // execute the desired SQL
    odbcDynamic.ExecuteSQL(_T(“SELECT * from UserMaster”));

    // retrieve the record array
    CODBCRecordArray* pODBCRecordArray = &odbcDynamic.m_ODBCRecordArray;

    // for every returned record…
    for (int iRecord = 0; iRecord < pODBCRecordArray->GetSize(); iRecord++)
    {
    CODBCRecord* pODBCRecord = (*pODBCRecordArray)[iRecord];
    CString strValue;

    // retrieve the desired column (by name)
    CDBVariantEx* pvarValue = NULL;
    if (pODBCRecord->Lookup(_T(“sUserId”), pvarValue))
    {
    // As shown in the example above, you can use the
    // CDBVariantEx::GetStringValue to have the value
    // translated into a CString and returned…
    pvarValue->GetStringValue(strValue);
    AfxMessageBox(strValue);

    // … or you can now use the appropriate CDBVariant member
    // variable to access the data. For example, if the column’s
    // data type is string, or text…
    AfxMessageBox(*pvarValue->m_pstring);
    }
    }

    That’s it! That’s how easy it is to interrogate any ODBC data source.
    The last thing that I will point out is that in the example above,
    I used my CDBVariantEx’s GetStringValue
    member function to retrieve the data in as a CString. However, because I chose to
    store the data in CDBVariant objects, you can also easily query that object as to
    the data’s exact type by inspecting the CDBVariant::m_dwType member variable. For more
    documentation on this small, but useful class, please refer to the Visual C++
    documentation.

    Downloads

    Download demo project – 15 KB
    Download source – 15 KB

    Date Last Updated: October 22, 2000

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